High Lockout
9B on Viessmann Vitodens 050-W

Your boiler has lost communication with one of its internal heat sensors, meaning it cannot monitor temperatures accurately to operate safely.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

Call an engineer soon

Book a Gas Safe engineer within 24 to 48 hours. Your boiler may be unsafe or could break down completely if left.

Technical description

Control mode: Sensor S3 lead break

What causes this fault?

This fault occurs when the boiler's control board loses its electrical connection to a specific internal temperature sensor. The most common cause is a failure of the sensor itself or loose, damaged wiring that prevents the boiler from safely monitoring the water temperature. Without this data, the system shuts down to prevent overheating.

Engineer required

This is a high severity fault. Stop using the boiler and call a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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Related fault codes

92View →93View →9AView →BCView →

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